Signal transmission system



Jan. 4, 1944.

Filed June 25, 1940 E. H. B. BARTELINK SIGNAL TRANSMISSION SYSTEM 4SBeejts-Sheej l s'ounce 0f SIGNAL POTENTIALS GRID POTENTIAL TOANODEVOLTAGE SOURCE 28 cur OFF 33 32 30 29 POTENTIAL GRID POTENTIAL A5FRACTION OF ANODE POTENTIAL.

TIME INITIAL Z7 NEGATIVE POTENTIAL Flg. 4,

'mia hi b'u'oalo 4- 6 8 I0 I2 FREQUENCY IN E KILOCYCLES GRID POTENTIALPULSE WIDTH IN PERCENT OF WAVE I.ENGTH 'N'biyian .1: .3 .4 .s .a .7.GRID BIAS 'IN PERCENT OF ANODE POTENTIAL (ti-5F 3.0F I

Fig. 2

CUT OFF FREQUENCY 2..OF FREQUENCY Inventor Everh ard H. BBartel in k,

H is Attqrney.

Jan- 4, 1944- E. H. B. BARTELINK SIGNAL TRANSMISSION SYSTEM Filed June25, 1940 '4 Sheets-Sheet 2 Fig.6.

SOURCE OF SIGNAL POTENTIALS POTE NTIAIJ- SOURC 5 or SIGNAL POSITIVEPULSE WAVE FORM NEGATIVE PULSE WAVE FORM Inventor Everhard H.137 Bartlin k,

. His Attorney.

1944- E. H. B. BARTELINK 2,338,395

SIGNAL TRANSMISSION SYSTEM Filed J une 25, 1940 4 Sheets-Sheet s SOURCE:jal.

$0 URCE OF A DDITIONA L POTE N TMLS SIG NA L P0 TENT! m2 I03 ")4 I05 4 Nn. ms

Inventor:

EverhaPd HBBartelink,

H is Attorney.

4, 1944- E. H. B. BART'E-LINK 3 5 SIGNAL TRANSMISSION SYSTEM 4Sheets-Sheet 4 Filed June 25, 1940 SOURCE 0F SIG/VAL POTENTIALS Fig.5.

SOURCE OF SIGNAL POTEIVT/A LS Q Flgl.

I I I l i9 n7 SOURCE OF SLOPE FREQUENCY FILTER MODULATED SIGNALInventor:

Everhard HBBartelink,

b .Nw? J y His Attorney.

Patented Jan. 4, 1944 2,338,395 SIGNAL TRANSMISSION SYSTEM Everhard H.B. Bartelink signor to General Elect tion of New York Nlskayuna, N. Y.,as-- rlc Company, a cpl-pota- Application June 25, 1940, Serial No.342,321 7 Claims. (Cl. 179-1715) My invention relates to the applicationof improved circuits, especially of circuits employing multivibrators,for use as part of a transmission. system or of a system used for thetransmission of controlling signals in power and other applications.

An object of my invention is to effect certain improvements inmultivibrators whereby their operation is less affected by interferingvoltages; whereby their utility with respect to the production ofcurrent impulses in definitely timed relation to other impulses, as, forexample, in television synchronizing systems, is improved; whereby theircontrol with respect to frequency and pulse width of the impulsesproduced is improved and facilitated; and whereby various otheradvantages, as hereinafter explained, may be secured.

In recent years multivibrators have come into common use particularly intelevision systems, where they have great utility. As so employed,however, they comprise electron discharge devices the grids of which arenormally without bias voltage or are biased negatively with respect totheir cathodes. I have found, however, in accordance with my invention,that very important improvements in the operation of such multivibratorsmay be secured by biasing the grids positively with respect to theircathodes. In fact marked improvements in all of the respects abovepointed out may be secured by such biasing of the multivibrator grids.The degree of such improvement is dependent upon the magnitude of thepositive bias potential. In most cases I have found a relatively highpositive bias potential to be desirable.

Further improvement is obtained by applying negative feedback to themultivibrator discharge devices.

It has been found in accordancewith my invention that if a bias besupplied to the grids of the discharge devices employed in amultivibrator which bias is positive with respect to the cathodesthereof, the frequency of oscillations produced thereby is thensubstantially linearly dependent upon the magnitude of such bias over avery wide range of frequency variation. An object of my invention is toutilize this property of such multivibrators for frequency modulation.

It is another object of my invention to utilize the properties describedabove for demodulation of a carrier wave. for the conversion of one typeof. frequency modulation into another, and for the timing of frequencymodulation signals, in general, for the interconversion of twodifferently characterized waves.

The novel features which are considered to be characteristic of myinvention are set forth with particularity in the appended claims. Myinvention itself, however, both as to its organization and method ofoperation together with further objects and advantages thereof may bestbe understood by reference to the following description taken inconnection with the accompanying drawings wherein Fig. l is adiagrammatic representation of a transmission system, employing amultivibrator in accordance with my present invention; Figs. 2 to 5 showcurves illustrating the operation of the system illustrated in Fig. 1;Fig. 6 illustrates another multivibrator system in accordance with myinvention; Fig. 7 shows curves illustrative of the embodiment of Fig. 6;Fig. 8 illustrates another multivibrator system in accordance with myinvention; Figs. 9 and 10 show curves illustrating the operation of thesystem of Fig. 8; Fig. 11 illustrates another multivibrator system inaccordance with my invention; Fig. 12 illustrates advantages obtained inthe use of my improved circuits; Fig. 13 shows curves illustrative ofthe system of Fig. 11; and Figs. 14, 15 and 16 illustrate furthermultivibrator systems in accordance with my invention.

Referring to Fig. 1 of the drawings, I have shown therein a transmissionsystem comprising an input circuit I, an output circuit 2, and amultivibrator 3 linking these circuits. A signal of a desired wave formis supplied to the input circuit I from a suitable source of signalpotentials as represented conventionally in the figure. This signal istransmitted through the multivibrator 3 and synchronizes the pulses inoutput circuit 2, in a definitely timed relation with the impressedsignal in input circuit I. The multivibrator comprises a pair ofelectron discharge devices 4 and 5 having the anodes connected to avoltage source (not shown) through anode resistors 6 and l. The controlelectrodes or grids 8 and 9 are respectively connected, throughcondensers l0 and II to the anodes of the opposite tubes, and the gridleaks I2 and I3 are respectively connected to the corresponding controlelectrodes or grids which through these grid leaks are connected with asource of bias potential which preferably is positive with respect tothe cathodes l4 and I 5. For this purpose a potentiometer I6 is providedconnected across the anode voltage source, and the grid leaks l2 and l 3are adjustably connected to the potentiometer through a decouplingresistor ,l I and a decoupling condenser l8. A positive bias voltage is.therefore impressed upon the control electrodes 8 and 9 which isvariable by adjustment of the movable contact l9 along the potentiometerl6.

In the operation of a multivibrator to produce spaced pulses thevoltages impressed upon the two control electrodes become periodicallyof such values that the anode current is interrupted, or blocked, theblocking occurring in the two tubes alternately. The negative chargeaccumulated on the condenser connected between a given control electrodeand the anode of the opposite discharge device at a given period in thecycle of operation and which is sufiiciently great to block thecorresponding tube, then leaks off in accordance with an exponential lawuntil anode current begins again to flow, or until the cutoil! point ofthe control electrode voltage-anode current curve of the dischargedevice is reached. The blocking condition is then removed and thecorresponding tube again becomes conductive.

In Fig. 2 for example, are shown a series of exponential curvesexpressing the relation between control electrode potential and timeunder difierent control electrode bias potential conditions, from zerobias potential, for which curve 20 illustrates the change in gridpotential with time, through successively greater positive bias producedby adjustment of the contact ill of Fig. l.

Exponential curves 2| to 25 illustrate the change in the controlelectrode potential for the latter positive bias conditions. Theportions of the curves above zero control electrode voltage can, ofcourse, only be realized while the cathode of the tube is not emittingelectrons as otherwise the control electrode conduction wouldappreciably alter their shape. As shown in Fig. 2, in curve 20corresponding to the zero bias condition, the slope at the cut-off point26, is considerably less than at the initial point 21. Throughout theseries of curves 2| to 25, however, the slope at the cut-off points, 28to 32, becomes progressively more nearly equal to the slope at theinitial point 21.

When as above described the applied positive voltage is madeprogressively higher, the exponential curves take the forms illustratedin Fig. 2 due to the different final unidirectional voltages to whichthe control electrode condensers would become charged. The occurrence ofcontrol electrode current, however, affects that part of a iven curvewhich lies above the zero control electrode voltage line, whereas it isthe region of the intersection of the curvewith the cut-off line, or theregion of negative control electrode voltages, which is of interest. Ifthe applied positive bias voltage is relatively high, as in the case ofcurve 25 of Fig. 2, the intersecting of the exponential curve with thecut-ofi line occurs in the initial portion 33, instead of in the curvedportion 34. Throughout this initial portion 33 the exponential curve issubstantially a straight line and its slope practically equals theinitial slope at initial point 21.

Referring to Fig. 3 in connection with Figs. 1 and 2' it will be seenhow the obtaining of increased stability against interference isobtained in a multivibrator system of the positive control electrodebias type as compared with the system of the zero control electrode biastype. Let it be assumed that numeral 35 designates a portion of acharacteristic curve 01' multivibrator control electrode voltageincluding the portion of the curve where the characteristic intersectsthe cutoff line. Numeral 36 designates the latter line. In the absenceof interfering voltages impressed on the control electrode circuit thetripping of the multivibrator will be. performed at regular timeintervals. TheintersectiQn 31 of control electrode voltagecharacteristic 35 and cut-ofi line 35 indicates the instant when, in theabsence of interference, the tripping occurs. Let it now be assumed thatinterference is introduced such as a voltage affecting the normalcontrol electrode voltage, and that the peak-to-peak swing of thisvoltage is a certain fraction, m, of the total driving peak-to-peakswing of the control electrode voltage. This interference may have anyarbitrary wave form, such for example as indicated by the curve 38.

Thus, under the influence of the interference, the effective drivingvoltage may be any single valued function which is included within theregion, m, around the normal or undisturbed characteristic 35, for anyone tripping of the multivibrator. For consecutive trippings, unless thefrequency of the multivibrator and of the interference are directmultiples, the phase of the interference may vary from one tripping ofthe multivibrator to another. The highest and lowest values of combinedor eifective control electrode voltages are given by the lines a and b,and their intersection with the line 36 determines the earliest andlatest times at which the multivibrator can trip.

Under these interference conditions the tripping operation may start notonly at the normal tripping moment indicated by the point designated bythe numeral 31 but because of the superimposed voltage represented by 38may start at any other instant either earlier or later, between theinstants t1 and t: on the cut-oil line 36. Designating this region asAt, the uncertainty region along the cut-off line, or that region at anypoint of which the tripping moment may occur, is expressed as The slopes of the normal characteristic is expressed by wherein y is theinstantaneous voltage expressed as a fraction of the peak-to-peakvoltage swing. In the cut-off region iLn dzm approximately, andtherefore s=g or At= g Therefore for a given interference theuncertainty region varies in extent inversely as the slope s of thecontrol electrode voltage characteristic. For example, if the controlelectrode voltage characteristic is, as indicated by the curve 39 ofFig. 3, of greater slope than as indicated by curve 35, other conditionsbeing substantially unchanged, the uncertaintyregion At for the voltagehaving characteristic 39, extending between ti' and t2, iscorrespondingly less than the uncertainty region At extending between t1and t2.

Thus when the slope of the control electrode voltage characteristic isincreased by impressing a positive bias voltage on the control electrodethe uncertainty range is decreased. Therefore the stability againstinterference of a multivibrator of the positive control electrode biastype is correspondingly greater than that of a multivibrator of the zerobias type. The amount of stability against external interference can becontrolled by the amount of control electrode bias used. This improvedstability is a major advantage which is obtained by using the improvedmultivibrator type in the transmission systems.

With regard to the simpler and more effective frequency controlobtainable in thepositive control electrode bias type of multivibratoras compared with the zero control electrode bias type, Fig. 2 showsthat, by varying the positive bias applied to the controlelectrodesiland9thereby changing the slope of the control electrodecharacteristic, the time elapsing, as the negative charge leaks off,from the initial point 21 of negative control electrode potential up tothe cut-off point is also varied causing a corresponding change in thefrequency of the oscillations produced by the multivibrator. Referringto Fig. 4, the latter figure shows the oscillation frequency as afunction of the positive control electrode bias ex pressed as a fractionof the anode voltage. As shown in Fig. 4, a practically linear relationexists between these quantities over a very considerable range ofoscillation frequency. Thus the multivibrator of the positive controlelectrode bias type makes possible, as compared to the zero bias type,the provision of a positive and simple control means for varying thefrequency, and since the frequency variation is controlled solely byvariation of direct current bias, it may readily be effected by remotecontrol means.

I have found that a lessened effect on the pulse width is produced whenthe frequency in the positive control electrode bias type ofmultivibrator is varied, as compared with the zero control electrodebias type. Referring particularly to Fig. 5, this figure shows the pulsewidth a a function of the frequency. Curves 40 and 4| illustraterespectively the pulse width and frequency characteristics of twopositive bias type multivibrators operating in different frequencyranges. As shown in the curves 40 and 4!, nearly all the change in pulsewidth is confined to the region 42 of low bias voltages.

Referring now to Fig. 6, a frequency modulation generator in accordancewith my invention is illustrated therein. The modulation generatorincludes a multivibrator in which a positive bias voltage is impressedon the control electrodes 43 and 44 of the multivibrator tubes 45 and 46from a suitable source (not shown) through a resistor 41 which alsoforms part of the anode circuit of a tube 48. Means for modulating thefrequency includes the above mentioned space discharge device or tube 48the anode-cathode space of which is connected between resistor 41 andground or the cathodes 49 and 50 of tubes 45 and 4B, and the controlelectrode cathode circuit of which is connected to a suitable source ofsignal potentials as represented conventionally in the figure.

The current in resistance 41, and therefore the voltage between theanode and cathode of discharge device 48, varies in accordance with thesignal, thereby varying the positive bias between control electrode andcathode. In this manner the frequency of oscillations produced by themultivibrator is caused to vary linearly with the signal applied to thecontrol electrode of discharge device 48, as may be seen by reference toFig.4.

The wave form of the multivibrator contains many harmonics which it isdesirable to filter out before the signal is transmitted, and for thispurpose a low pass filter is provided in the outquency modulation overa' band of a width F.

the low pass filter may be arranged to have a cut-off frequency F0 ofapproximately 3.5 F. The

multivibrator maybe adjusted to operate at a frequency of 2.5 F when nomodulation is present. In this particular case, therefore, the frequencyis swung, by the varying modulation voltage impressed on themultivibrator control electrodes, between the limits of approximately2.0

F and 3.0 F which corresponds to .57 and .86 of the filter cut-offfrequency F0. This is a range of approximately .30 F0 or .40 F. If themultivibrator wave is perfectly symmetrical, as it can be made to be bysymmetrical construction, the wave will not contain any even harmonics,and it is suflicient if the low pass filter eliminates the thirdharmonic of the lower multivibrator frequency. Thus in this case themultivibrator can be swung from approximately .40 F0 to .90 F0. Thiscorresponds to a range of .50 F0, or approximately seventy-five percentof the average fre quency of the carrier.

The resulting signal from the multivibrator of Fig. 6 is impressed upona space discharge device 52, which changes the frequency, by harmonicamplification, or by heterodyning with a second wave impressed thereon,to that of the desired frequency band. It is to be noted that since themultivibrator system described in connection with Figs. 6 and '7 iscapable of transmitting a very wide range of frequencies, the system isespecially useful for wide-band frequency modulation and as such isapplicable even to wideband frequency modulation for televisionpurposes. I

The multivibrator utilized for frequency modulation in accordance withmy invention and as shown in Figs. 6 and 7 possesses the importantadvantage that the range over which the frequency of the carrier wavemay be varied without objectionable distortion may be a very large partof the carrier frequency itself. This means that the modulation may beeffected at a low carrier frequency thereby to reduce the extent towhich the undesired variations in the carrier frequency itself affectsthe frequency of the finally transmitted carrier. This low frequencycarrier may then, after having its frequency modulated in accordancewith the signal, be heterodyned to a desired high frequency. Of coursethe final frequency is then affected by undesired variatio'ns infrequency of the carrier source used to produce the heterodyning actionbut this source may be crystal controlled, or otherwise accuratelyregulated, to minimize such variations. Thus undesired variations of thefinal carrier may be very small.

This is indistinct contrast to more conventional systems in which theinitial frequency modulation without distortion occurs only over a verysmall percentage of the carrier frequency. This necessitates the use ofa high carrier frequency for the initial modulation whose frequencycannot be stabilized by crystal control and I the undesired variationsof which may, therefore, be a substantial part of the desired variation.

Referring to Fig. 8, the arrangement shown therein is one which, amongothers suitable for the same purposes, is particularly suitabl for usein electronic gear, or frequency changing, systems, square wavegenerators, timing units, and similar applications. In this arrangement,

put circuit of tube 48. Referring more particuin one multivibrator stageIt thereof each tube of the pair of multivibrator tubes It and II, whichare of the positive control electrode bias typ is provided wtih acathode resistor of relatively low resistance. Each cathode resistor iscommon to the anode circuit and control electrode circuit of thecorresponding tube. The circuit of cathode 56 of tube 5| includesresistor 51 and the circuit of cathode ll of tube I5 includes resistor59. These cathode resistors serve the purpose of impedance matching.They serve further for the injection of synchronizing pulses andbuffering, while they also introduce negative feedback and thusstabilize the multivibrator operation.

As is well known, it is of advantage to use low impedance transfercircuits, such as provided by theuse of the cathode resistors I! and 59,between the successive stages of an electronic gear because' lowimpedance transfer circuits reduce the difilculties due to crosstalk andmake it possible to monitor the wave forms in the system withoutdisturbing the operation thereof. A further considerable advantage liesin the fact that the use of low impedance output circuits eliminates thenecessity of employing a voltage divider in the multivibrator outputcircuit. A further important advantage lies in the fact that thenegative feedback provided by a cathode resistor stabilizes themultivibrator frequency against variations in the supply voltages andalso assists in limiting the peak cathode currents. Multivibrators havecommonly been subject to two types of instabilities, i. e., the trippingtime of the pulse has varied erratically causing uneven spacing of thepulses, and a slow drift of the pulse frequency has occurred. I havefound that the erratic variation of the tripping time: is madenegligibly small by impressing in the control electrodes a biaspotential which is positive with respect to the cathodes, and'that thefrequency drift is obviated by the negative feedback provided by thecathode resistors 51 and 59. Using both of these means, the timing ofthe pulses is caused to be substantially exactly uniform during theentire period of operation of the multivibrator.

These advantages, above pointed out in connection with the use of thecathode resistors 51 and 59 in Fig. 8, hold as well for all othermultivibrator circuits described hereinafter in which cathode resistorsmay be used, as for the multivibrator system of Fig. 8.

As low impedances, constituted by resistors 51 and 59, are used in thecathode circuits of multivibrator tubes 54 and 55, it is obvious thatonly small voltages can be generated across these impedances. However,since the synchronization of multivibrators requires-only small amountsof voltage, therefore the impressing of a voltage across the cathoderesistor offers a solution well adapted to theproblem ofsynchronization.

In regard to the buffering action of the cathode resistors 51 and 59,since these cathode impedances are of low value, any load thereacrossdoes not materially affect the frequency of operation of themultivibrator 58. It is thus possible in general to utilize the voltagesfrom the cathode impedance for the synchronizing of any multivibratorstage. or for the driving of other circuits.

Still further advantages are obtainable by the use of the cathoderesistors in multivibrator circuits, as may be more readily seen from aconsideration of the curves shown in Figs. 9 and 10. In thoseapplications of multivibrator circults in which synchronization isrequired, it is as a rule advantageous that the synchronizing pulseshave a very steep leading edge intheir wave forms. In previous types ofmultivibrators it has been necessary to derive these pulses from theanode circuits. The positive wave forms in the anode circuits, asrepresented in Fig. 9 for one of a pair of multivibrator tubes, show arounding oil, at ll, of the leading edge of the wave form. The negativevoltages, represented by Fig. 10, obtained on the anode of the othermultivibrator would, of course, require inversion before beingapplicable for synchronizing purposes, The rounding off of the wave formas shown in Fig. 9 is caused by the sudden discharge of the couplingcondenser 8|, shown in Fig. 8 which is connected to the anode of onemultivibrator tube 54 of a pair, through the control electrode tocathode discharge path 62 of the other multivibrator tube II, when thecontrol electrode 63, of this other tube suddenly becomes positive. Nowsince a cathode resistor, 58, is inserted in the cathode circuit 'of thesecond tube 55, the condenser discharge current occurs as an additionalcurrent in this cathode circuit. Theadditional current therefore causesan extra peak of current on the cathode wave form. The presence of thisextrapeak generally constitutes an advantage in the use of these pulsesfor synchronizing, or for the generation of square waves, and similarpurposes. The special advantages which are obtained in the use inparticular of the square wave generator will be explained hereinafter.

Referring further to Fig. 8, in this figure is illustrated also theinterconnection of two multivibrator stages in the system. The lowimpedance transfer circuit for connecting the stages includes thecathode resistor 59 of first stage tube 85, cathode resistor ll of thesecond stage tube 85, and a connection means or network 66interconnecting the resistors 58 and 64 at suitable points thereon toproduce the desired transfer voltages. It may be of advantage to employa network including a copper oxide rectifier 81 or other suitablerectifier in this transfer circuit or to insert a resistor 68 in thisconnection. The network may be arranged to reduce any feedback ofvoltages, generated in the second multivibrator stage 69, into the firststage I. It can be shown that it is of advantage if the product of therectifier capacitance and its resistance in the reverse currentdirection is smaller than l/2 in, where is is is the higher of the twofrequencies employed in the electronic gear unit comprisingmultivibrator units 53 and 69.

Referring to Fig. 11, another multivibrator circuit is illustratedtherein adaptable for use in electronic gear systems, time bases, squarewave generators, and similar circuits. While for reasons of stabilityagainst interference and protection against locking in of incorrectfrequency division ratios it is preferable to use positive bias in themultivibrator, the circuit will also operate when the applied bias isreduced to zero or even if it is made slightly negative. The circuit ofFig. 11 may comprise a multivibrator stage including four spacedischarge devices or tubes which may be of the multiple grid, screengrid, pentode or triode type. Instead of the single multivibrator tubes,two twin tubes of the multiple grid, screen grid, pentode or triode typemay be employed. In one of the preferred forms, as illustrated in Fig,11, two twin tubes 10 and II of the triode type are employed, eachincluding two space discharge devices. In tube 18 one discharge device12 comprises anode 18, control electrode 14 and cathode l8 and the otherdischarge device 18 comprises anode TI, control electrode 18, andcathode 18. In tube ll, one discharge device 80 comprises anode 8|,-control electrode 82 and cathode 88, and the other discharge device 84comprises anode 85, control electrode 86, and cathode 81.

The above described arrangement, employing for the fourth dischargedevice either a pentode or a triode, having the cathode 88, of themultivibrator discharge device 8| and the cathode 81 of the fourthdischarge device connected to ground through a common cathode resistor88 and having the control electrode 88 of the fourth discharge deviceconnected to ground, is my preferred arrangement used for the generationof square topped waves.

In the circuit shown in Fig. 11 the injection of the synchronizing pulseis accomplished from a low impedance network 88 connected to a suitablesource of signal potentials. Elimination of feedback from themultivibrator stage into pre' ceding stages is obtained by injecting thesynchronizing pulses into the control electrode of one of the spacedischarge devices, as control electrode 14 of the first discharge device12 of twin tube 10. The cathode 15 of this first discharge device oftwin tube 10 is directly connected to the cathode 18 of the seconddischarge device 18 of twin tube Ill. The two cathodes l and 18 of thetwin tube 18 have a common resistor 90, which may connect these cathodesto ground. Thus the synchronizing pulses, injected into the controlelectrode-cathode circuit in-'- cluding the common thode resistor 88,are transferred to or imprssed upon this cathode resistor 80 whichforms? part of the cathode circuit of the second discharge device 16.

Likewise the cathode 83 of space discharge device 88 of twin tube H isconnected to the cathode 81 of discharge device 84 of the latter twintube, and the two cathodes 88 and 81 have the common resistor 88, whichmay connect them to ground. The multivibrator stage comprising spacedischarge devices 78 and so of tubes and H respectively then operates inessentially the same manner as the multivibrator stage 53 describedabove in connection with Fig. 8. As in the case of stage 58 of Fig. 8,output voltages from the multivibrator stage of Fig. 11 may be obtainedfrom the cathode resistance 88, of a. second multivibrator tube H.

In order to synchronize a second stage 8!, partially shown in thecircuit illustrated in Fig. 11, the synchronizing pulse may be deriveddirectly from the cathode resistor 88 or from a tap 82 on the resistor.Monitoring of the synchronizing pulses may be accomplished in thecathode circuit including resistor 88 without disturbing the operationofthe system provided the impedance of the monitoring circuit (notshown) is large relatively to the impedance of the cathode resistor 88.Safe monitoring without interruption of operation may be obtained byinserting a series resistor (not shown) between the cathodes 83 and 81and the connection point of the monitor.

The anode 85 of the space discharge device H which is the finaldischarge device of the unit constituted by twin tubes 18 and II may beemployed to obtain a buffered output from the unit, the output voltagesbeing obtained between terminals 83 and 84. By suitable arrangementobtained by providing a condenser between the anode 85 of tube H andground.

In regard to the square wave output, the arrangement in the circuit ofFig. 11 is such that a short positive pulse appears on the commoncathode resistor 88 of the third and fourth space discharge devices 88and 84.. A positive voltage. of the cathode 81 with respect to groundmeans a negative voltage of the control electrode 88 with respect to thecathode 8'l,' if this control electrode be connected to ground. In Fig.11 this connection of control electrode 88 to ground is efiected througha resistance 88 which may be very small. 'By suitable choice of theoperating characteristics of the fourth space discharge device 84, thenegative pulse may be caused to drive the fourth discharge device 84beyond cutoff. This may be accomplished in the case of a triode, such asdischarge device 84, by the provision of alarge decoupling resistor 88in its anode circuit. As the discharge device 84 is thus driven beyondcut-oil, the upper or peaked portion of the pulse wave form, designatedby the numeral 81, is therefore clipped oil and a square wave positiveoutput, of wave form designated by the numeral 88', is obtained in theanode circuit of the fourth space discharge device. 84.

In both of the multivibrator arrangements of the positive bias typeabove described in connection with Figs. 8 and 11, the same generaladvantages of increased stability, easier. adjustment of frequency, andreduction of the change of pulse width with frequency as were previouslydescribed herein, are obtained. These forms of my invention aredisclosed in all essential particulars and are claimed in my applicationSerial No. 473,360, which is assigned to'the assignee of my presentapplication.

A specific advantage which is obtained in the use of multivibrators ofthe positive bias type in electronic gear or frequency changing systemsmay be easily understood by reference to Fig. 12. The numeral 88designates a curve of multivibrator control electrode voltage havingsynchronizing impulses impressed thereon and having the low degree ofslope characterizing the control electrode voltage in multivibrators ofthe zero bias type. Numeral I designates a corresponding curve carryingthis synchronizing frequency but having the steep slope characterizingthe control electrode voltage in the positive bias type ofmultivibrator. In this figure it is shown how the increase in steepnessof the multivibrator control electrode wave form characterizing thepositive bias type produces a greater voltage range for discriminationagainst synchronization of the multivibrator over incorrect frequencyratios. Vp in the illustrated case is the relatively large voltage rangeavailable in the positive bias type for discrimination against incorrectsynchronizing ratios, and V2 is the relatively small voltage rangeavailable in the zero' bias type.

In Fig. 13 are illustrated. wave forms as they are applied to themultivibrator circuit of Fig. 11 in order to obtain a combined mixingand square-wave-forming, or clipping, operation. Additional voltagesfrom a suitable source repa square wave resented conventionally in thefigure are applied at terminals IOI and 84 to the impedance 95 connectedbetween the control electrode 86 of the space discharge device II andground. These additional voltages may be so poled and of such magnitudeas to drive the control electrode 88 of this discharge device II beyondcut-off for periods other than those prescribed by the voltagesappearing on the common cathode resistor 88. In Fig. 13 is illustratedone example in which a complete pedestal pulse as used in television,may thus be derived from a multivibrator circuit, assumed to beoperating at the horizontal frequency of a television system, byinserting a voltage, corresponding to the vertical pedestal pulse, in anegative polarity into the control electrode 86,

In this case, waves which may be of the form designated by the numeralI02 are impressed upon the grid 82 of discharge device 'II. Wavesdesignated by the numeral I03, corresponding to the waves designated byI02, appear in the cathode resistor 88, and pulses designated by thenumeral I04 appear across the grid 86 and cathode 87. The additionalvoltage, applied to the grid 86 across the resistor 85, has the waveform designated by the numeral I05. The mixing of this wave I05 with thepulses I04 appearing across resistor 88 results in the pedestal pulse I00 in the anode circuit including anode 85 and cathode 81. It will beunderstood that if, in obtaining the pulse I06 of square form, aclipping operation is required as in the case of the example illustratedin Fig. 13, this may be accomplished in the manner above explained inconnection with Fig. 12.

Other effects may be obtained by suitably changing the operatingconditions of the fourth discharge device 85. I

In Fig. 14 is illustrated a multivibrator circuit of the positive biastype similar in general to the circuits of Figs. 8 and 11 and adapted tothe same uses as the circuits of the latter figures. In the circuit ofFig. 14 pentode multivibrator tubes I01 and I08 are employed, and forthe injection of synchronizing pulses a suppressor grid, as grid I09 oftube I01, is utilized, connected to a suitable synchronizing-pulsesource.

In Fig. 15 is illustrated a multivibrator circuit of the positive biastype in which the pulse width may be changed by an improved methodemploying a pure direct current control. In this circuit the rate ofchange of the voltages on the control electrodes H0 and III of the tubesH2 and I I3 respectively may be varied independently, for example, bypotentiometers H4 and H5, while one combined control means, as thepotentiometer II6, varies the rate of change on both control electrodessimultaneously and thus controls the multivibrator frequency. Thisarrangement obviates the need for making variable any of the signalcarrying components of the multivibrator circuit.

The circuit of Fig. 6 has been described herein as a frequencymodulation generator. It will be understood that the circuit of Fig. 6is a special case of the more general case where the multivibrator orrelaxation oscillator of which the multivibrator is a special case, isusedas a device in a transmission system to correlate a pair of variablevoltage functions. One of these voltage functions may be a variation inthe operating, or bias, voltage of the multivibrator while the other maybe a frequency modulation of a carrier. It is well known that it ispossible to change the timing intervals or frequencies of amultivibrator, or, of a' relaxation oscillator in general, by

changing its direct current operating conditions.

Conversely, and in agreement with the reci- 5 procity theorem, upon theoccurrence of any change in the timing intervals or the frequency of amultivibrator or of a. relaxation oscillator in general, changes areproduced in its direct current operating condition. This process isreferred to as frequency demodulation. This possibility of frequencydemodulation exists because the properties of the multivibrator orrelaxation oscillator circuit are such that the circuit tends to follow,or to become synchronized with, any impressed external signal.

The property of a multivibrator, or, in general, of a relaxationoscillator, of synchronizing its oscillations in accordance with animpressed external signal may be utilized to obtain frequency changingalso. The frequency changing may be obtained with either an unmodulatedwave or afrequency modulated wave. For example, the relaxationoscillator may be synchronized to a frequency one-fourth that of theimpressed signal. If the fifth harmonic of this frequency be nowfiltered out, a frequency is thereby obtained which, is at all times 20%higher than the original input signal. This relation will hold for thecase wherein the signal is unmodulated and the signals are thereforecharacterized by their frequencies alone, and the relation also holdsfor the case wherein the original signal has been frequency modulated.

In this process of frequency changing, the frequency of the impressedinput signal and the frequency of the output signal obtained from theoscillator are integral multiples of a predetermined frequency which inthe present case is the oscillator synchronizing frequency. The processmay, therefore, be referred to as frequency transformation on a commonmultiple basis.

In the circuit of the special case of a device for correlating a pair ofvoltage functions, illustrated in Fig. 6, the multivibrator of positivebias type is described as employed to convert an audio frequency into afrequency modulated signal. The possibility of thus obtaining frequencymodulation, as in Fig. 6, by usingthe positive grid type ofmultivibrator exists because the relation between the positive bias andthe frequency of the multivibrator is practically linear.

It will be noted further that since, when the frequency of themultivibrator is varying, its D. C. operating conditions will also vary,therefore any varyin in frequency of impressed signal will also cause avarying of the D. '0. potential across the resistors 6, 'I and II inFig. 1, resistors I2I, I29 and I30 in Fig. 8, resistors I22, I3I, andI32 in Fig. 11, resistors I24, I33, I34 in Fig. 14, resistors I28, I29,I35 and I36 in Fig. 15, as well as in all of the cathode resistors shownin these circuits. In other words, frequency demodulation occurs inthese circuits in which the multivibrator frequency varies. Across allof these elements in Figs. 1, 8, 11, 14 and 15 the frequency demodulatedsignal is available in greater or smaller amplitude, and accordingly thefrequency demodulated signal may be derived from the oscillationgenerator employed in the circuits illustrated in any of Figs. 1, 8, 11,14 and 15.

It will be noted also that the circuits illustrated in Fig. 1, in eachof sections 53 and 88 of Fig. 8, in Fig. 11, in Fig. 14, and in Fig. 15are above explained, for example, in connection with.

frequency transformation on a common multiple basis. For this reason thecircuits of Figs. 1, 8, ll, '14 and are all suitable for changing thefrequency of a frequency modulated signal from one range into anotherrange.

In Fig. 16 is illustrated the application of an improved multivibratorcircuit of the positive bias type to use asa limiter in a frequencymodulation receiving system. In this system a multivibrator oi thelatter type comprising, for example, two tubes H1 and H8 is connected atits input side to a source of amplitude modulated signals which maycomprise tuning circuits (not shown) of the system, and at its outputside to the slope filter H9 of the system. The slope filter is a devicewhich will convert a signal having frequency modulation and no amplitudemodulation into a signal having both types of modulation, but in whichthe amplitude modulation is materially in greater proportion than thefrequency modulation.

As is well known, the voltage and the wave shape of the multivibratoroutput vary only very slightly if the operating frequency ofthe-multivibrator is changed over relatively wide ranges under theinfluence of an external synchronizing force. This output is also widelyindependent of the amount of input voltage. These properties of themultivibrator exactly meet the requirements to be fulfilled by a limitercircuit. An additional advantage lies in the fact that, in performingthe limiting process, the multivibrator permits the obtaining ofamplification rather than causing a loss as in the case of certain othertypes of limiters. The amplification function is inherent in themultivibration circuit, in that this type of circuit is by nature anoscillator in which a small amount of synchronizing voltage is able tocontrol the frequency of oscillation. The employment of themultivibrator of the positive bias type, illustrated in Fig. 16 forexample, as the limiter in frequency modulation systems results ingreatly improved stability, ease of Irequency control and otherimportant advantages thereby making more practicable the employing ofthe multivibrator type of limiter, with its general advantageshereinbefore described, in place of limiters of the types heretoforeused.

In regard to the converting of direct to alternating current, it will beunderstood that any of the modulated multivibrator systems describedhereinabove, in accordance with my present invention, may be used'forcurrent conversion, the arrangement being such that the alternatingcurrent output is synchronous to a small external alternatingcontrolling voltage.

The multivibrator employed as the limiter for frequency modulatedcarrier waves is disclosed and claimed in my copending applicationSerial No. 474,114, which is assigned to the assignee of my Presentapplication.

My invention has been described herein in particular embodiments forpurposes of illustration. It is to be understood, however, that theinvention is susceptible of various changes and modifications and thatby the appended claims I intend to cover any such modifications as fallwithin the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A transmission system for producing a frequency modulated signal,said system comprising a multivibrator having cathodes and controlelectrodes, a source of amplitude modulated signals, means to impress abias potential positive with respect to said cathodes upon said controlelectrodes, and means to vary said potential in accordance with saidamplitude modulated signals, said bias potential being of sufficientlyhigh value to cause substantial increase in the rate of change of thecontrol electrode potential at each cycle thereof over what would be therate of change under zero or negative bias conditions thereby todecrease the varying of the tripping ,time by interfering voltages andto increase the stability of said system against said interferingvoltages.

2. A transmission system for producing a frequency modulated signal,said system including a source of amplitude modulated signals, ,amultivibrator having cathodes and control electrodes, a source of anodepotential for said multivibrator, means to impress a varying potentialpositive with respect to said cathodes upon said control electrodes,said means including a resistance and an electron discharge devicehaving its anodecathode circuit in series therewith connected to saidpotential source, and means including said first-named source to impresssaid amplitude modulated signals upon' the control electrodecathodecircuit of said discharge device .to vary the impedance thereof inaccordance with said first-named signal, said positive bias potentialhavin-g'a value sufficiently high to increase substantially the rate ofchange of the multivibrator control electrode potential over thecorresponding rate of change which would occur under zero or negativebias conditions thereby to increase substantially the stability of saidsystem against interfering voltages.

3. A transmission system for producing a frequency modulated signal,said system including a source of amplitude modulated signals, amultivibrator having cathodes and control electrodes, a source of anodepotential for said multivibrator, and means to impress upon said controlelectrodes a bias potential positive with respect to said cathodes andvarying in accordance with said amplitude modulated signals, said meansincluding in series with said anode potential source a resistance and animpedance connected to said first-named source and adapted to be variedin accordance with the signals therefrom, and means to connect saidcontrol electrodes to said resistance, said positive bias potentialbeing of sufficiently high value to cause the rate of change of controlelectrode potential to be substantially increased compared with the rateof change which would occur under zero or negative bias conditionsthereby to increase substantially the stability of the system againstinterfering voltages.

4. The combination, in a wide band frequency modulation system, of amultivibrator comprising a pair of electron discharge devices, eachhaving a control electrode and a cathode, means to supply a potential tosaid control electrode positive with respect to said cathodes andvarying in continuous increments in accord with desired signals therebyto vary the fundamental frequency of said multivibrator over a widerange in linear relation to said potential, and means to eliminateharmonics of the fundamental frequency of said multivibrator thereby toproduce a sinusoidal carrier wave varying in frequency substantiallylinearly over a wide range in accord with said signals.

5. The combination, in a wide band frequency modulation system, of amultivibrator comprising a pair of electron discharge devices, eachhaving a control electrode and a cathode, means to supply a potential tosaid control electrodes positive with respect to said cathodes andvarying in con-.

tinuous increments in accord with desired signals thereby to vary thefundamental frequency of said multivibrator linearly over a wide rangewith said potential, said multivibrator having an output circuitincluding a filter for eliminating'harmonies of the fundamentalfrequency of said multivibrator, and means to translate the range offrequencies produced at the output of said filter to a range offrequencies higher in the frequency spectrum.

6. The combination, in a wide band frequency modulation system, of asource of potential variable in continuous increments of both frequencyand amplitude over a wide range in accord with desired signals to betransmitted, a multivibrator comprising a pair' of electron dischargedevices, each having a control electrode and a cathode, means to supplypotential to said control electrodes positive with respect to saidcathodes, and means to vary said positive potential in accord with saidcontinuous variations of said first mentioned source thereby to vary thefundamental frequency of the oscillations produced by saidmultivibrator, the average fundamental frequency of said multivibratorbeing higher than the highest frequency of said source and saidfundamental frequency of said multivibrator varying linearly in accordwith said positive potential over a wide range of frequencies higherthan the highest frequency of said source.

'I. The combination, in a'wide band frequency modulation system, of amultivibrator comprising a pair of electron discharge devices, eachhaving an anode, a cathode, and a control electrode connected insymmetrical multivibrator circuit arrangement to eliminate evenharmonics of the oscillations produced by said multivibrator, means tosupply potential to said control electrodes positive with respect tosaid cathodes and varying continuously over a range in accord withsignals to be transmitted thereby to vary the fundamental frequency ofoscillations produced by said multivibrator in substantially linearaccord with said signals, a low pass filter, means to transmit theoscillations of said multivibrator I through said low pass filter, saidfilter having a cutoff frequency sufficiently high to pass the highestfundamental frequency of said oscillations and sufiiciently low toeliminate the third harmonic of the lowest fundamental frequency of saidoscillations, and means to transmit the range of frequencies appearingat the output of said low pass filter.

EVERHARD H. B'. BAR'I'ELINK.

CERTIFICATE OF CORRECTION.

Patent No. 2,558,595. January L 19th.

EVERHARD H. B. BARTELINK.

It is hereby certified that error appears in the printed specificationof the above numbered patent requiring correction as follows: Page 1;,first column, Iine L for "wtih" read --with--; page 7, second column,line 51, claim 2, strike out "first-named signal" and insert insteadamp1itude modulated signals-; and that the said Letters Patent should beread with this correction therein that the same may conform to therecord of the case in the Patent Office. 7

Signed and sealed this inn day of April, A. D. 191m.

Leslie Frazer (Seal) Acting Commissioner of Patents.

